introduction to system dynamics

7/30/2019 Introduction to System Dynamics

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Introduction to

System

DynamicsIshardita P. Tama


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Outline

Understanding Systems Thinking

System Dynamics (SD)

Causal Loop Diagram Stock and Flow Diagram

References


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System thinking

is the process of understanding how things, regarded assystems, influence one another within a whole

an approach to problem solving, by viewing "problems" as

parts of an overall system, rather than reacting to specificpart, outcomes or events and potentially contributing tofurther development of unintended consequences


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Und

erstandingSyste

ms


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California 1990 :o ZEV (zero emission vehicle)electric carsno tailpipe, no

pollution

o Looks good?

o


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Yesterdays solution becomes todaysproblem


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Feedback

o The more NATO nuclear weapons, the more Soviet Unionbuilt

o The more a firm lowers its price, the more the competitors

respond in kind, forcing the firm to lower even moreo The less nicotine in a cigarette the more smokers must

consume to get the dose they need

o The more attractive a city, the greater immigration fromsurrounding areas, leading to high criminal rate, and

eventually decreasing its attractiveness


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We have to be aware and understand theimplications of feedback by the decisions we make.

Learn about the structure and dynamics ofcomplex system

System Dynamics


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System Dynamicso Introduced by Jay Forrester of MIT in 1958

o The study of information-feedback characteristics of

industry activity to show how organizational structure,amplification (in policies), and time delays (in decisionsand actions) interact to influence the success of theenterprise (Forrest 1958 & 1961)


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High AbstractionLess Details Macro

Level Strategic Level

Middle AbstractionMedium Details

Meso Level TacticalLevel

Low Abstraction

More DetailsMicro Level

Operational Level Individual objects, exact sizes, distances, velocities, timings,

Discrete Event Entities (passive

objects)

Flowchartsand/or transportnetworks

Resources

Aggregates, Global Casual Dependencies, Feedback Dynamics,

Mainly discrete

System Dynamics

Levels (aggregates)

Stock-and-Flow Diagrams

Feedback loops

Mainly continuous


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Tools Causal Loop Diagram

o a useful way to represent dynamic interrelationships

o Provide a visual representation with which to communicate that

understanding

o Make explicit one's understanding of a system structure - Capture the

mental model

Stock and Flow Diagramo Develop model using stock and flow principle


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Causal Loop Diagram a useful way to represent dynamic interrelationships Provide a visual representation with which to communicate that understanding Make explicit one's understanding of a system structure - Capture the mental model

Variables - an element in a situation which may act or be acted upon Vary up or down over time (not an event) Nouns or noun phrases (not action words)

Links / Arrows - show the relationship and the directionof influence between variables

S's and O's - show the way one variable moves or changes in relation toanother S stands for "same direction O stands for "opposite direction

orB - Balancing feedback loop that seeks equilibrium

orR - Reinforcing feedback loop that amplifies change


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Structure of CLD

HungerAmount

EatenB

O

S

The O means that thetwo variables move inopposite directions, allother things being equal

The S means that thetwo variables move in thesame direction, all otherthings being equal

Nature of loop. Indicateswhether the loop is

balancing or reinforcing

Causal relationshipbetween two variables

Variables


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Employee

Performance

SupervisorsSupportiveBehavior

Unsupportive

Behavior

Structure

S

S

Reinforcing Loop

Perf.Level

Time

Behavior Over Time

SupportiveBehavior

Employee

Performance

SupervisorsSupportiveBehavior


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Discrepancy

Inventory

Adjustment

Structure

ActualInventory

DesiredInventory

Desired Inventory

Time

Behavior Over Time

100

100 - -

100 ++

Actual Inventory

S

S

S

O

Balancing Loop


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CLD mental model

Kalkulasi ?

Stock and Flow Diagram (SFD)o Quantification issue


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Stocks Stocks are accumulations.

Stocks hold the current state of the system: whatyou would see if you were to take a snapshot of thesystem. If you take a picture of a bathtub, you caneasily see the level of the water. Wateraccumulates in a bathtub. The accumulatedvolume of water is a stock.

Stocks fully describe the condition of the system at

any point in time. Stocks, furthermore, do notchange instantaneously: they change graduallyover a period of time.


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Flows / Rate Flows do the changing.

The faucet pours water into the bathtub and thedrain sucks water out. Flows increase or decreasestocks not just once, but every unit of time. The

entire time that the faucet is turned on and thedrain unplugged, water will flow in and out.

All systems that change through time can berepresented by using only stocks and flows.


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STOCK

FLOW / RATE

Auxiliary

Stock

Rate

i1

i2

i3

Auxiliary

o1

o2

o3


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Birth rate norm

Death rate norm

Population

Birth rate

Death rate

P

BRN

DRN

BR

DR

Births

population

Deaths

Birth rate normal

Death rate normal

R

B

+

+

+

+

+

--


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References Todd Little, Introduction to Systems Thinking and

Causal Loops,www.toddlittleweb.com/Presentations/wwsdc/SystemsCausalLoops.ppt

Erik R Larsen, Introduction to System Dynamics,www.cs.unibo.it/~fioretti/AC/AC2005/docs/Bertinoro.ppt

J.D. Sterman, Business Dynamics

A.C. McLucas, System Dynamics application
http://www.toddlittleweb.com/Presentations/wwsdc/SystemsCausalLoops.ppthttp://www.toddlittleweb.com/Presentations/wwsdc/SystemsCausalLoops.ppthttp://www.cs.unibo.it/~fioretti/AC/AC2005/docs/Bertinoro.ppthttp://www.cs.unibo.it/~fioretti/AC/AC2005/docs/Bertinoro.ppthttp://www.cs.unibo.it/~fioretti/AC/AC2005/docs/Bertinoro.ppthttp://www.cs.unibo.it/~fioretti/AC/AC2005/docs/Bertinoro.ppthttp://www.toddlittleweb.com/Presentations/wwsdc/SystemsCausalLoops.ppt

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